Wear resistant cobalt bonded tungsten carbide composite



July 8, 1969 CYCLES WEAR WEAR RESISTANT COBALT BONDED TUNGSTEN CARBIDECOMPOSITE R. S. MONTGOMERY Filed Oct. 17, 1966 PLATED WITH GOLD UNPLATEDCARBIDE COEFFICIENT OF FRICTION UNPLATED PLATED WITH GOLD INVENTORROBERT S. MONTGOMERY CYCLES United States Patent ce 3,454,375 WEARRESISTANT COBALT BONDED TUNGSTEN CARBIDE COMPOSITE Robert S. Montgomery,Princeton, N.J., assignor to Ingersoll-Rand Company, New York, N.Y., acorporation of New Jersey Filed Oct. 17, 1966, Ser. No. 587,202 Int. Cl.C23b 5/00; C23c 13/02; C22c 29/00 U.S. C]. 29191.2 6 Claims ABSTRACT OFTHE DISCLOSURE The wear resistance of cobalt bonded tungsten carbidecomposites is improved by coating the surface of the composite with athin layer of either gold, tin, manganese, titanium or silver.

This invention relates to improvements in the wear resistance ofcemented carbide bodies and more particularly to tungsten carbidecomposites.

Cemented carbide bodies or tungsten carbide composites are made up of aplurality of tungsten carbide granules which are bonded together bycobalt or a material primarily made up of cobalt. It has been found thatwhen cemented carbide bodies of this type are subjected to repeatedsurface stresses, such as provided by sliding or rolling contact, aftera few cycles the cobalt binder pulls away from the tungsten carbidegranules. There is a failure of the bonding between the cobalt andtunsten carbide granules. This allows large flakes of the carbidecomposite to break away from the body. When enough of these flakes breakaway, the body is no longer usable for its intended purpose. Large pitsare formed in the surface of the body thus increasing the coefiicient offriction and eventually resulting in complete failure. Because failureoccurs after a few cycles, the use of cemented carbides wherever cyclicstresses of great numbers are encountered has been limited. The strengthof tungsten carbide composites would be advantageous in many moreapplications where cyclic stresses occur such as in high pressurecompressors where pistons and cylinders are in sliding and sealingcontact with each other if the stability of composites subjected tocyclic stresses could be increased.

It is the principal object of this invention to provide an improved wearresistant material.

It is another object of this invention to provide a wear resistantmaterial with increased stability when subjected to repeated cyclicstresses.

It is another object of this invention to provide a method of improvingthe wear resistance of cemented carbide bodies.

In general, these objects are carried out by coating a cemented carbidebody with a material selected from the group consisting of gold, tin,titanium, manganese and silver.

The foregoing and other objects will become apparent from the followingdescription and drawings in which:

FIG. 1 is a graph showing the coefficient of friction of this inventionwhere the ordinate axis is a logarithmic scale and abscissa axis is alineal scale; and

FIG. 2 is a graph showing the improved wear qualities of this inventionwhere both the ordinate and the abscissa are logarithmic scales.

By this invention it has been discovered that the amount of surfacefatigue on a tungsten carbide composite can be reduced over a prolongednumber of cyclic stresses by coating the composite with a thin layer ofa material containing a metal selected from the group consisting ofgold, tin, manganese, titanium and silver.

The base material is a standard tungsten carbide com- 3,454,375 PatentedJuly 8, 1969 posite usually made up of tungsten carbide particles orgranules bonded together with cobalt. The cobalt may be in the range ofthree to twenty-five percent by weight of the composite with the grainsize of the tunsten carbide varying according to desired hardness andstrength.

The composite is then coated by a suitable method such as electroplatingwhich per se is well known or by vapor deposition. Vapor depositionusually involves placing the tungsten carbide composite in a vacuum andevaporating the coating metal onto the carbide. This process is per seold and is used for achieving thin coatings of materials which do notlend themselves to electroplating. Because vapor deposition may be usedfor all of the coating metals, it is a preferred method of coating. Whendeposition is used, the coating is not firmly fixed to the base but theresults achieved by the end product are the same as if firmly fixed. Itis intended that other methods of plating are within the scope of thisinvention, and in some cases, it may be possible to simply paint thecoating material on the carbide.

The coating thickness is important to the extent that if it is too thin,it will give no improved results whereas if it is too thick, the excesswill merely wear away giving no improved wear until a thin coating isachieved. The coating material, by itself, has poor wear qualities sothat a thick coat actually decreases wear qualities until it has wornaway. With tin it was found that a coating thickness of one-halfmicroinch failed to achieve any improved wear qualities in the carbidecomposite while a coating of 2 microinches worked well in improving wearqualities. With gold, a .8 microinch coating was found to be effective.Although these thicknesses are difficult to measure, it is believed thatabout one-half microinch is the thinnest effective coating. A thicknessof .0001 inch was found to be the upper limit. In some applications thismay be too thick while in other applications a thicker coating may bedesirable.

If the end product is to be used where size is a critical factor, athinner coating is preferred. For example, if a tungsten carbidecomposite is to be used for the piston of a high pressure compressor,size is critical as there must be a seal achieved between the piston andcylinder walls. If a thick coating is used which wears away quicklybefore the increased wear qualities are apparent, the sealing betweenthe piston and cylinder will be lost. The loss of sealing between thepiston and cylinder due to wear of the carbide composite is one of thereasons coating is necessary. Therefore, if the seal is lost due to toothick a coating, no advantage is achieved.

In tests performed it was shown that the tendency of the carbide to failwas greatly decreased when the coating was used. In all tests thecarbide used was a particular grade of General Electric carbide. Thedifferences in grade result from different tungsten carbide grain sizewith all grades tested having about six percent by weight cobalt as abinder.

The tests were performed by using a General Electric grade 895 carbideflat plate. The flat was securely mounted and a hemispherical carbiderider of either General Electric 895 carbide or General Electric 883carbide was used. A reciprocating wear machine was used which applied aload of 33.2 pounds and the rider moved ovr the flat at an average speedof one foot per second. The actual distance of travel in a completecycle was 8 inches with a time of travel of A second. The temperature ofthe working surface was maintained at 65 C.

In a pilot test using an unplated General Electric grade 895 flat and aGeneral Electric grade 895 rider, it was found that the coefiicient offriction was about .2. After cycles, the coefficient of friction was .3and at 600 cycles, the coefiicient of friction was about .45. This isshown generally on FIG. 1 With a solid line.

The wear increased greatly as the number of cycles increased. Wear, inthis test, is a measure of the amount of material removed from thehemispherical rider in cubic millimeters. In the pilot test using aGeneral Electric 895 flat and the same material rider, the weargradually increased until about 500 cycles and then the materialexperienced complete failure. This is shown by the solid line on FIG. 2.

In general, the coatings of this invention do not improve thecoefficient of friction. As indicated on FIG. 2, when the carbide wasplated with gold, the coefficient of friction was slightly improveduntil about 500 cycles. At this point the coefficient of friction becameabout the same as the uncoated carbide. After the point where theunplated carbide fails, the coefficient of friction of the coatedcarbide remains substantially constant. This is simply because of lackof failure and not per se due solely to the coating as would be the caseif the coating acted simply as a solid lubricant. The coating thicknessWas .0001 inch.

When the fiat was coated with gold containing 24% cobalt, the wearqualities of the composite improved greatly. Until about 500 cycles, thewear of the plated material is the same as that of the unplatedmaterial. However, from that point, as shown on FIG. 2, the wearcontinues to be gradual rather than abrupt. At 10,000 cycles completefailure has still not occurred whereas, at 500 cycles the unplatedmaterial has completely failed. Pure gold produced test resultssubstantially similar to the gold with cobalt added but it was foundthat the addition of cobalt to the gold increased hardness and waseasier to work with and more readily obtainable.

The remainder of the coating materials produced similar results withsimilar curves. However, some of the coating material tried did notwork. While tin Worked, indium, which in most cases is similar to tin,did not work. If the coating acted as a solid lubricant, if tin Worked,indium would also work. However, the wear qualities decreased andfailure occurred faster than without the coating. Other factors, besidesthe general lack of improvement of the coefiicient of friction otherthan after failure occurred, which indicated that the coating was notacting as a solid lubricant are the fact that thinner coatings are usedin this invention than would work with solid lubricants. What materialswill improve wear qualities is not predictable to the extent that theeffectiveness of solid lubricants is predictable.

Those materials which worked in addition to gold were tin, titanium,manganese, and silver which all delayed failure until approximately10,000 cycles and in some cases, failure never occurred. Silver works tothe extent that it delayed complete failure until about 10,000 cycles,but in the region of mild wear, the wear was greater than the unplatedtungsten carbide composite. If desired, cobalt may be added to the othercoating metals but this Was found not as useful as in the case of gold.

It has been found that those metals which do achieve the desired resultare all soluble in cobalt with the exception of silver. Each of themetals tends to stabilize the alpha phase cobalt. As is known at highertemperatures cobalt has one crystalline structure usually designated thealpha phase. When pure cobalt is cooled to about 600 C. it changes to asecond crystalline structure. By stabilizing the alpha phase, thecoatings of this invention lowers the temperature at which thecrystalline structure changes. This is believed to be one reason why thesubject metals improve the wear qualities of tungsten carbidecomposites. A corollary to this theory is that the coating relieves thestresses in the outer surface of the cemented carbide.

Silver is different from the other metals which work. Unlike the othermetals, silver is not soluble in cobalt. In the region of mild wear, thewear is not improved yet failure is prevented. The lack of improved wearin the region of few cycles is attributed to the lack of solubility ofsilver in cobalt.

Each of the metals that work has an atom size which is larger thancobalt and each is softer than cobalt. Not all metals whose atom size islarger than cobalt will work. In addition to larger atom size, it isbelieved that the metal must be soluble in cobalt, with silver as anexception. Although aluminum conformed to these requirements, it did notwork probably owing to its reactivity toward oxygen.

It has also been found that those metals which work or increase the wearqualities may be alloyed with metals which do not have this effect andthe overall wear qualities are increased. By the term alloys of thosemetals, I do not intend to limit myself to interalloys of the fivemetals that work but include alloys of the preferred metals with othermetals that do not work. Even though the materials that work arecombined with other materials, those that work still coat the carbidecomposite to an extent sufficient to improve Wear. For example, it wasfound that bronze, a tin containing alloy of copper, worked. The metalsthat do work may be alloyed with each other and work. Although all testswere carried out using tungsten carbide, it is contemplated that thiscoating will improve wear when used with other carbides of heavy metalssuch as titanium.

From the foregoing it can be seen that an improved tungsten carbidecomposite has been provided by coating its surface with a materialselected from the group consisting of gold, tin, titanium, manganese andsilver and alloys thereof.

It is intended that the foregoing be merely that of a preferredembodiment and that the invention be limited only by that which iswithin the scope of the appended claims.

I claim:

1. A wear resistant material comprising tungsten carbide granules bondedtogether with cobalt; the surface of the composite formed by saidgranules and cobalt being coated with a material soluble in cobalt.

2. A wear resistant material according to claim 1 wherein the coatingmaterial is softer than cobalt.

3. The material of claim 1 wherein the coating material has a thicknessof between .0000005 inch and .0001 inch.

4. A wear resistant material comprising a plurality of granules of acarbide of a heavy metal bonded together with cobalt; the surface of thecomposite formed by said granules and cobalt being coated with amaterial selected from the group consisting of gold, titanium, tin,manganese and alloys of those metals.

5. The material of claim 4 wherein the coating material has a thicknessof at least .0000005 inch.

6. A wear resistant material comprising a plurality of granules of acarbide of a heavy metal bonded together with cobalt; the surface of thecomposite formed by said granules and cobalt having a coating of silverless than .0001 inch thick.

References Cited UNITED STATES PATENTS 1,910,884 5/1933 Comstock 29182.1X 1,984,203 12/1934 Sieger 29182.1 2,313,070 3/1943 Hensel et al.29-182.1 2,313,227 3/1943 De Bats 29-1821 X 2,439,570 4/1948 Hensel etal. 29-182.1 2,731,711 l/1956 Lucas 29--182.8 3,153,163 10/1964 Foldeset al 29-182.8 X 3,165,822 1/1965 Beeghly 29182.7

L. DEWAYNE RUTLEDGE, Primary Examiner.

E. L. WEISE, Assistant Examiner.

US. Cl. X.R. 29-182. 1, 182.8,

